Polyesters have long been known and are usually obtained by polycondensation of polyhydric alcohols with polyfunctional carboxylic acids. Recently, stiff-chain and liquid-crystalline aromatic polyesters have also become known. It is also known that these polymers can considerably improve the mechanical properties of matrix polymers when mixed therewith. For example J. Appl. Polym. Sci., Volume 43, 157-173 (1991) describes mixed fibres made from polyethylene terephthalate (PET) and a thermotropically liquid-crystalline p-hydroxybenzoic acid/2,6-hydroxynaphthoic acid polyester. When this polyester is mixed with the PET fibre polymer, the result is an improvement in the mechanical properties such as the tensile stress and elastic modulus. However, the proportion of this polymer needs to be at least 10% by weight in order to improve the mechanical properties of the resulting mixed fibres.